GI Tract Secretion Flashcards
What is GI function co-ordinated by?
integration of neural and hormonal signals
What type of signals stimulate GI function?
physical (stretch receptors)
chemical (sights and presence of food)
What are the 3 main phases that regulate GI function?
- cephalic
- gastric
- intestinal
Draw a diagram showing the links between the three phases. Highlight both the feed forward and feed backward mechanisms
What is the cephalic phase intiated by?
taste, thought, smell of food
What is the most important regulator of the cephalic phase and what is it triggered by?
Brain and higher iontegrating centres increase vagal activity which intiates the parasympathteic branch of the ANS
What are the effects of the PSNS on the cephalic phase
- increase salivary secretion
- increase secretion in the stomach
- HCl
- Mucus
- Pepsin
- Gastrin
- Stimulation of bile ducts, hepatocytes and pancreatic acinar cells
What occurs as a result of increased secretion in the stomach?
increased blood flow to stomach
blood factors (e.g. insulin and glucagon)
What is the gastric phase initiated by?
food entering the stomach causing distension and release of gastrin - especially coffee, alcohol, calcium
What are the 4 main inputs into G cells of the stomach during the gastric phase?
- Parasympathetic pathways
- Decreased acidity to stomach due to buffering of food
- distension of antrum
- proteins, peptides and amino acids
What is released by G cells of the stomach and what are their primary targets?
Gastrin
- Gastric motility
- Parietal cells
- Trophic maintenance of GI epithelium (cell turn over)
What do parietal cells release that act as a negative feedback mechanism
HCl
What is the intestinal phase intiated by?
presence of food in the duodenum
What is the requirement for the release of gastrin?
If pH >3, duodenal peptides/amino acids cause gastrin release
What is the requirement for gastric inhibition and intestinal stimulation?
If pH < 2
What causes release of GIP and CCK?
duodenal fats and breakdown products
What causes secretin release
acid entering the intestine
What is secretion?
Addition of substances (fluids, enzymes, ions etc) into the lumen of the gastrointestinal tract
What are the main producers of secretions?
- Produced by salivary glands (saliva)
- Cells of the gastric mucosa (gastric secretion)
- Exocrine cells of the pancreas (pancreatic secretion)
- Liver (bile)
What are the main characterisitics of saliva?
- High HCO3-
- High K+
- Hypotonic
- α-amylase and lingual lipase
Factors that increase the secretion of saliva
PSNS (primary) and SNS
What factors decrease the secretion of saliva?
- sleep
- dehydration
- atropine
What are the main functions of saliva?
- Initial digestion of starches and lipids (very little)
- Dilution and buffering of ingested foods
- Protection of teeth and gums
- Lubrication of ingested foods with mucous (mucin)
Describe the location and cell type of sublingual glands
deep in floor of mouth, underneath tongue, mostly mucous cells
Describe the location and cell type of subandibular glands
under lower edge of mandible, mixed glands containing serous and mucous cells
Describe the location and cell type of parotid glands
below ear and over the masseter (jaw muscle), serous cells secreting an aqueous fluid composed of water, ions and enzymes
Describe stage 1 of salivation
- Acini cells secrete 1° secretion (isotonic)
- Na+, Cl-, K+, HCO3-
- Amylase and mucin production
Describe stage 2 of salivation
Myoepithelial cells stimulated by neural input
Describe stage 3 of salivation
- Duct cells - 2° medication
- Reabsorb Na+ and Cl- and add K+
- HCO3- concentration is altered depending on flow rate
Affect of high flow rate on saliva?
increased HCO3-
Affect of low flow rate on saliva?
more HCO3- extracted
Describe saliva in the mouth in comparison to the plasma?
hypotonic (more dilute)
Draw a diagram indicating how salivary secretion is regulated
What is the main function of oesophgeal secretion?
secretes mucous for lubrication/protection (no real digestive function)
What is the action of simple mucous glands in the main body of the oesphagus?
protects against mechanical damage
What is the function of compound mucous glands at the gastric end of the oesophagus?
protect agaonst chemical damage
What are the 4 main types of gastric cells in the stomach and give their exact location
- parietal cells - body
- chief cells - body
- G cells - antrum
- mucous cells - antrum
Action of HCl
Protein digestion (a bit) – mostly for pepsinogen activation at acid pH, creates pH~2
Action of pepsinogen
protein digestion (a bit)
Action of intrinsic factor
vitamin B12 absorption (in the ileum) – really important
action of mucus
protection and lubrication
What are the three main chemicals of gastric secretion
- HCl
- Pepsinogen
- Intrinsic Factor
What factors increase gastric secretion?
- Gastrin
- Acetylcholine
- Histamine
- Parasympathetic
What factors decrease gastric secretion?
- H+ in stomach
- Chyme in duodenum
- Somatostatin
- Atropine
- Cimetidine
- Omeprazole
Describe HCl secretion on the apical surface
- H+ is secreted into the lumen via the H+-K+ ATPase
- Cl- follows by diffusion through an apical channel
Describe HCl secretion on the basolateral surface
- HCO3- is exchanged for Cl- via the chloride-bicarbonate exchanger (alkaline tide)
- Eventually HCO3- is secreted back into the GI tract in pancreatic secretions
How much does the cephalic phase contribute to secretion?
and what is it triggered by?
30% of secretion
smell, taste, chewing, swallowing, conditioned reflex in anticipation of food
What is responsible for stimulation of the cephalic phase?
- Direct stimulation of the parietal cells by the vagus
- Indirect stimulation of the parietal cells by gastrin
- Vagal gastrin releasing peptide (GRP) stimulates gastrin release from G cells. Gastrin hormone enters the circulation and stimulates parietal cells to release HCl
What causes the gastric phase and what percentage does this contribute to secretion?
60% secretion
Distension of the stomach and presence of breakdown products of proteins, amino acids and small peptides in the stomach
What does distension of the stomach cause?
o direct vagal stimulation of parietal cells
o indirect stimulation via gastrin
o local reflexes in the antrum that stimulate gastrin release
What are the direct effects of amino acids and small peptides on G cells?
stimulates gastrin release
What percentage is the intestinal phase of secretion and what is it triggered by?
10% of secretion
presence of breakdown products in the duodenum
When is HCl secretion inhibited?
when HCl is no longer needed to convert pepsinogen to pepsin. This occurs after the chyme moves into the small intestine and the H+ buffering capacity of the food is no longer a factor
What is the direct pathway of somatostatin?
bindstoreceptorsonparietalcells (and inhibits adenylate cyclase via Gi protein so inhibits HCl release)
What is the indirect pathway of somatostatin?
inhibitshistaminereleasefrom stomach and gastrin release from G cells so HCl inhibited
What is responsible for pepsinogen secretion?
secreted by chief mucous cells in the oxyntic glands in response to vagal stimulation
H+ triggers local reflexes which stimulate chief cells to secrete pepsinogen
draw a diagram detailing the conversion of pepsinogen to pepsin
How much does the exocrine pancreas secrete into the duodenum per day?
~1L of fluid
What is the composition of the fluid secreted into the duodenum by the exocrine pancreas
Composed of an aqueous solution containing enzymes and high HCO-3
What is the function of HCO3-
to neutralise stomach H+
Characterisitcs of pancreatic secretion
- High HCO-3 (isotonic)
- Pancreatic lipase, amylase and proteases
Factors that increase pancreatic secretion
- Secretin
- Cholecytokinin (CCK) (potentiates secretin)
- Parasympathetic supply
What is the innervation of the exocrine pancreas?
- PSNS = vagus stimulates secretion
- SNS = inhibits secretion
How are pancreatic enzymes stored?
Stored in condensed zymogen granules until release
Where is pancreatic secretion released from and describe the composition of this secretion
Aqueous component of secretion released from centroacinar cells and ductal cells
- Pancreatic fluid is an isotonic fluid containing Na+, K+ Cl-and HCO-3
- Modification of the (ion) composition by the ductal cells results in a fluid secretion rich in HCO-3
Where is the enzymatic component of pancreatic secretion released from and what is its composition?
- Pancreatic amylase and lipases are secreted as active enzymes
- Pancreatic proteases are secreted in an inactive form and activated in the duodenum
Draw a diagram detailing what is released from different cells in pancreatic cells and the ion exchnage that takes place
What are the three types of pancreatic enzymes?
- pancreatic amylase
- pancreatic lipase
- proteolytic
action of pancreatic amylase
polysaccharide –> disaccharide
much more important than salivary amylase
action of pancreatic lipase
triglycerides –> monoglycerides and fatty acids
very important source of lipase
Action of proteolytic enzymes
cleave proteins at different sites –> amino acids and small peptides
What are the three main types of pancreatic proteolytic enzymes?
- Tripsinogen –> trypsin
- Chymotrypsingogen –> Chymotrypsin
- Procaboxypeptidase –> carboxypeptidase
All inactive to prevent self digestion
Draw a diagram showing the activation of proteolytic enzymes
Describe the regulation of pancreatic secretion in the cephalic phase of digestion
mediated by the vagus nerve ( mainly enzymatic secretion)
Describe the regulation of pancreatic secretion in the gastric phase of digestion
initiated by distension of the stomach and mediated by the vagus nerve ( mainly enzymatic secretion)
Describe the regulation of pancreatic secretion in the intestinal phase of digestion
accounts for 80% of pancreatic secretion and both enzymatic and aqueous secretions are stimulated
Pancreatic Response of acinar cells in the intestinal phase of digestion
- Duodenal I cells secrete CCK in response to the presence of amino acids, small peptides and fatty acids in the intestinal lumen
- Vagal release of ACh potentiates CCK action
- Triggers acinar cells to produce enzymes
Pancreatic Response of ductal cells in the intestinal phase of digestion
- Secretin released by the S cells of the duodenum is the major stimulus for aqueous rich HCO-3 secretion
- Secretin release is triggered by the arrival of acidic chyme in the duodenum
- ACh and CCK potentiate secretin action
- Triggers ductal cells to produce Na+, K+ Cl- and HCO-3
What are the 3 main functions of the gall bladder?
- Stores bile which is continuously produced by the hepatocytes and flows to the gallbladder through the bile ducts
- Concentrates bile
- Epithelial cells lining the gallbladder absorb ions and water iso-osmotically
- Ejects bile
- Begins ~30 minutes after a meal. The major stimulus for ejection is the release of cholecystokinin from the I cells in the duodenum and jejunum
What is bile?
- Essential for the digestion and absorption of lipids
- Bile salts emulsify lipids to prepare them for digestion and solubilise the products of digestion into ‘packets’ called micelles
- Mixture of bile salts, bile pigments and cholesterol Bile is not enzymic
Describe the regulation of bile secretion
- • In between meals
- Bile salts recirculated to liver via the enterohepatic system more bile produced
- Bile stored / concentrated in gall bladder
- Cephalic phase (just before a meal)
- Neural (parasym) stimulation via vagus nerve to ↑ bile flow
- During/after a meal
- Chyme in duodenum stimulates release of CCK & secretin
Action of CCK?
- triggers release of stored bile
- gallbladder contraction
- sphincter of oddi relaxation
Action of secretin
- triggers bile secretion
- especially NaHCO3-
Draw a diagram indicating the effect of chyme in the duodenum on bile secretion
Outline the secretions of the small intestine
- Crypts of Lieberkühn
- Located over the surface of the SI
- Crypts lie between intestinal villi
- Crypts and villi covered with two types of cells
- goblet cells – secrete mucus
- enterocytes – in crypts secrete water and electrolytes (~1800ml/day)
- in villi absorb water and electrolytes (along with products of digestion)
- Exact mechanism for secretion still not known –involves active secretion of Cl- and HCO3- into the crypts at apical membrane. Electrical gradient causes Na+ to be drawn though and together, the osmotically active particles draw water though by osmosis.
- Regulated largely by distension and tactile or irritative stimuli from chyme
Outline the secretions of the large intestine
- Alkaline mucus
- High[K+]&[HCO3-]
- No digestive enzymes
- Functions:
- protection & lubrication
- neutralisation of H+ produced by gut bacteria
- Greatest trigger is distension / mechanical stimulation of walls
- ↑ secretion triggered by acetyl choline & vasoactive intestinal peptide (VIP)
- ↓ secreVon triggered by adrenaline & somatostaVn
- Some parasymp input - Extreme parasymp stimulationexcess mucus - defection (e.g., stress, nervousness)
Main stimulus for bile secretion
- secretin
- CCK (gall bladder contraction: relaxation of sphincter of Oddi)
- Some central and enteric nerve input
